SPR sensing has become a widely used technique for the measurement of biomolecular interactions, quantification of proteins, and measurement of DNA. Briefly, SPR relies on an optical excitation of a charge-density oscillation existing at the interface between a thin metallic film and a dielectric material. Resonance conditions are achieved when the light is in total internal reflection at a wavelength-angle couple matching a wavevector of the Surface Plasmon (SP). Multiple optical configurations can possibly excite SPs.
A popular configuration uses monochromatic light to interrogate an angle of resonance with the SP, commonly known as the Kretschmann configuration. Many commercially successful SPR instruments are based on this Kretschmann optical configuration. However, this technology suffers from drawbacks limiting its use in biomedical applications; such SPR instruments are usually expensive to implement, they cannot be deployed on the field due to size constraint of the optical path, and they are not compatible with biological samples. Thus, in spite of the popularity of the Kretschmann SPR instruments, there still exists a need to develop a SPR instrument combining the high resolution of the angle interrogation configuration with the advantages of an inexpensive and portable instrument.
SPR instruments based on different configurations have been investigated as alternatives to the angle interrogation SPR configuration. Among them, a SPR instrument using a fiber optic as the sensing element is a cost effective alternative to research grade instruments; they are portable and can be adapted to various applications such as salinity sensor, biosensor for wound healing, biosensor for cardiac markers, and biosensor for staphylococcal enterotoxin B. Sensitivity of fiber optic SPR can be improved using near infrared excitation of a micro prism located at the tip of the fiber optic. However, the resolution achieved with fiber optic SPR is limited by the numerical aperture (NA) of the optical fiber required to implement fiber optic SPR. A large numerical aperture (NA=0.39) fiber is used to propagate the SPR-active wavelength-angle couples. However, due to a large number of wavelength-angle couples propagating in the fiber optic and entering in resonance with the SPR surface, the SPR spectrum broadens resulting in a limited resolution characterising this configuration. To minimize this effect, a low numerical aperture (NA=0.12) fiber can be modified with a micro-prism at the distal end thereof to improve the SPR spectrum and increase the accessible range of refractive indices. Using the fiber optic SPR configuration, the resolution is limited to approximately 1.4×10−6 RIU (Refractive Index Unit). Further reduction of the numerical aperture of the optical fiber can achieve a resolution similar to the angle interrogation SPR configuration (approximately 5×10−7 RIU). However, current manufacturing techniques do not enable such low numerical aperture.
An alternative to angle interrogation SPR or fiber optic SPR uses a multi-wavelength excitation. This configuration combines elements of the angle interrogation SPR and fiber optic SPR instruments. In a multi-wavelength excitation scheme, collimated white light from an excitation optical fiber is reflected at a single angle and the reflected light is analyzed with a spectrophotometer using a collection optical fiber. Among other factors, the resolution of multi-wavelength SPR is limited by the spectral resolution of the spectrophotometer, which is a function of the grating density. The recent development of a miniature spectrophotometer with high spectral resolution may potentially enable the measurement of the refractive index with high resolution using SPR with a small footprint. In the case of the angle interrogation SPR configuration, the resolution depends on scanning of the incident angle (slow measurement and complex mechanical setup) or on focusing of the incident light beam at the interface between a prism and a thin metallic film (made for example of Au) onto a linear array of photodiodes (precise alignment and lengthy optical path are required for high resolution). Hence, the angle interrogation SPR configuration is not suitable for portability and for an inexpensive design of SPR instrument. A current drawback limiting the use of a multi-wavelength SPR instrument is the precise alignment of the optics at the angle of excitation or the manufacture of a small sensing element.
Increasingly, the need for multiplex arrays is arising for simultaneous multi-analyte detection. Spatially resolved SPR measurements provide a technology for monitoring local changes of refractive index on a surface. Thus, the detection of biomolecular interactions for multiple systems/replicates is possible on a spatially resolved sensing array. SPR imaging, also called SPR microscopy, has been applied for high-throughput analyses of biomolecular binding event. SPR imaging methodology has also been recently optimized by improving resolution, optical coupling, and protein array formation. However, no SPR measurement presents the dual capability of measuring the conventional SPR response and the SPR image of a surface.